We have continued our studies of paramagnetic iron and copper-containing proteins primarily utilizing optimal and various paramagnetic resonance spectroscopies. For low spin ferric heme proteins, we are able to define the structures of all heme ligand combinations endogenous to heme proteins using EPR. For mammalian hepatic cytochrome P-450, for example, we are able to magnetically characterize the particular cytochrome in a heterogenous mixture that is associated with aryl hydrocarbon hydroxylase activity and will assay for this material in tissue. We are studying the effects of alternative structures of imidazole and will show how these are related to different functional states in hemoglobin, tryptophan oxygenase and other hemoproteins, using nuclear modulation spectroscopy. With the linear electric field effect (LEFE) in EPR, we will study asymmetric paramagnetic metal centers and will use this technique for both the structural characterization and an analysis of bonding parameters.